"We will not ignore attempts to interfere with our democratic processes," Carter said. Asked later to elaborate, Carter said he was referring to Russia's "interference in the internal affairs of nations, short of war".

President Barack Obama's homeland security adviser said on Wednesday that it would be very hard to hack into America's voting systems in a way that could alter an election outcome.

Lisa Monaco said election systems were not connected to the internet and were diffusely operated by state and local governments.

"That makes it extremely disparate, extremely diffuse and, as a consequence, extremely difficult to have an effect across the board that would result in a change in results," Monaco said.

The bigger worry, she said, involved efforts to sow "concern or confusion" about the resilience of the system.

Monaco's remarks echoed those of FBI Director James Comey, who told an audience last week that "the vote counting in this country tends to be kind of clunky," which makes it harder for hackers to infiltrate.

Earlier this year, Republican party presidential candidate Donald Trump dismissed suggestions that Russia had sought to influence the US election by engineering the theft of the Democratic Party emails.

Representatives from Clinton's campaign claimed Russians hacked computers belonging to the party, and had released the emails on the eve of the party's convention. They also pointed out that Trump had, in the past, expressed admiration for Russian President Vladimir Putin.

The cyber leak showed Democractic officials scoffing at Clinton's former rival, Bernie Sanders and his supporters, and prompted Debbie Wasserman Schultz to resign as Democratic National Convention chairwoman.

Trump has downplayed his relationship with Putin, but says he wants to improve relations with Moscow.

Shyla Nelson, a cofounder of the civil rights group Election Justice USA, told Al Jazeera: "The narrative that somehow 'foreign operatives' would be doing so [hacking] in order to interfere with our national elections is not only unsubstantiated, it's reckless and incendiary."